1. Field of the Invention
The present invention relates both to a thermal infrared detector having a high fill factor and a construction in which a photosensitive area that receives infrared light is held above the substrate with a space interposed by beams, i.e., having a thermal isolation structure, and to a method of fabricating the detector.
2. Description of the Related Art
Various configurations have been proposed for improving the fill factor of a thermal infrared detector having a thermal isolation structure, including the configuration of the infrared ray solid-state imaging device disclosed in Japanese Patent Laid-open Publication No. 209418/98 by Kimata et al. and the configuration of the thermal infrared detector array disclosed in the paper by Ishikawa et al. (xe2x80x9cLow-cost 320xc3x97240 uncooled IRFPA (Infrared Focal Plane Array) using a conventional silicon IC processxe2x80x9d; SPIE Vol. 3698, 1999, pp. 556-564). Referring to FIG. 1, the two-dimensional infrared solid-state imaging device described in Japanese Patent Laid-open Publication No. 209418/98 is shown as an example of a prior-art thermal infrared detector having a thermal isolation structure. FIG. 1 shows a sectional view taken along the current path in one picture element of the two-dimensional infrared solid-state imaging device.
First, regarding the thermal infrared detector shown in FIGS. and 2, a concavity that is to become cavity 104 is formed on the surface of silicon substrate 100 as shown in FIG. 1. Beams 102 and 103 composed of dielectric films 108 and 109 laminated on the surface of silicon substrate 100 overlay cavity 104. Each of dielectric films 108 and 109 is several hundred nanometers thick, and beams 102 and 103 are approximately 1 xcexcm thick, i.e., the sum of the thicknesses of the dielectric films on silicon substrate 100. The width of each of beams 102 and 103 is on the order of 1-3 xcexcm.
Each of beams 102 and 103 supports thermal detector 105, which includes thermistor bolometer thin-film 101, and holds thermal detector 105 above cavity 104. Each of these dielectric films 108 and 109 is composed of a material such as a silicon nitride film or a silicon oxide film having high thermal resistance, and each dielectric film controls the flow of heat from thermal detector 105 to silicon substrate 100. These two dielectric films 108 and 109 constitute the mechanical structure of beams 102 and 103 and thermal detector 105 and support thermal detector 105.
Metal wiring 106 and 107 is formed between dielectric films 108 and 109. One end of each of metal wiring 106 and 107 is connected to thermistor-bolometer thin-film 101. The other end of metal wiring 106 is electrically connected to signal line 202, which is provided on silicon substrate 100 as shown in FIG. 2, by way of contact 110 that is formed on dielectric film 109. Signal line 202 provided on silicon substrate 100 has been omitted in FIG. 1. The other end of metal wiring 107 is electrically connected to signal read-out circuit 201 by way of contact 111 formed on dielectric film 109. In other words, thermistor-bolometer thin-film 101 is electrically connected to signal read-out circuit 201 by way of metal wiring 106 and 107 and contacts 110 and 111. Signal read-out circuit 201, which is provided in silicon substrate 100, is omitted in FIG. 1.
Infrared ray absorbing part 112 is joined to the surface of thermal detector 105 that is directed away from cavity 104 by way of junction pillar 113. Infrared ray absorbing part 112 is a component for absorbing infrared rays and converting these rays to heat, and is constituted by a silicon nitride film or a silicon oxide film, or by a lamination of these films. Junction pillar 113 both keeps infrared ray absorbing part 112 separated from thermal detector 105 and thermally links infrared ray absorbing part 112 and thermal detector 105. Similar to infrared ray absorbing part 112, junction pillar 113 is constituted by a silicon nitride film or a silicon oxide film, or by a lamination of these films. The dimensions of junction pillar 113 are preferably, for example, several xcexcm thick and 1-2 xcexcm long, and junction pillar 113 may be of any shape.
Upon irradiation of infrared rays onto the infrared ray absorbing part in a thermal infrared detector, the infrared rays are absorbed into the infrared ray absorbing part, causing the temperature of the infrared ray absorbing part to rise. The infrared rays that have been irradiated upon the infrared ray absorbing part are then detected by sensing the temperature change of the infrared ray absorbing part. The thermal infrared detector of the prior art shown in FIGS. 1 and 2 is thus mainly constituted by, infrared ray absorbing part 112 and thermal detector 105. In this thermal infrared detector, the temperature change brought about in infrared ray absorbing part 112 by the infrared rays that are incident to infrared ray absorbing part 112 are conveyed to thermal detector 105 by way of junction pillar 113. The change in temperature of infrared ray absorbing part 112 is then detected by detecting change in the characteristics of thermal detector 105 that is caused by the temperature change, which in concrete terms is the change in electrical resistance of thermistor-bolometer thin-film 101 shown in FIGS. 1 and, 2.
FIG. 2 shows one entire picture element 200 and one portion of signal read-out circuit 201. Signal read-out circuit 201 that is established in picture element 200 is constituted by a MOS transistor or a diode. Contact 205 is formed in this signal read-out circuit 201. This contact 205 is connected by way of metal wiring 204 to contact 206, which is formed in control clock bus line 203. Control clock bus line 203 is provided for controlling signal read-out circuit 201. Metal wiring 106, on the other hand, is connected to signal line 202 by way of contact 110.( signal line 202 is provided for reading out signals from thermal detector 105.
FIG. 3 shows another example of a thermal infrared detector described in Japanese Patent Laid-open Publication No. 209418 in which thermal detector 301 is arranged above, and separated by the distance of cavity 302 from, silicon substrate 300. Thermistor-bolometer thin-film 303 is provided in thermal detector 301, thermistor-bolometer thin-film 303 being surrounded by dielectric protective films 304 and 305. Thermal detector 301 is supported above silicon substrate 300 by beams 306 and 307.
Thermistor-bolometer thin-film 303 is connected to a signal read-out circuit (not shown in the figure) in silicon substrate 300 by metal wiring 308 and 309, each of which is formed for carrying current, and contacts 311 and 312 formed on dielectric protective film 305 and dielectric film 310. Metal wiring 308 and 309 is enclosed by dielectric protective films 304 and 305.
A photosensitive area, which is composed by sandwiching infrared ray absorbing part 315 between metal reflecting film 313 and infrared ray absorbing film 316, also made of metal, is link e d by way of junction pillar 314 to the surface of thermal detector 301 that is directed away from silicon substrate 300. Junction pillar 314 is formed as a single unit with metal reflecting film 313. Infrared ray absorbing part 315 and metal infrared ray absorbing film 316 are laminated in that order on the surface of metal reflecting film 313 that is directed away from junction pillar 314. A three-layer optical resonation structure is thus constituted by metal reflecting film 313, infrared ray absorbing part 315, and metal infrared ray absorbing film 316.
If xcex is the wavelength of the infrared light that is to be detected by the thermal infrared detector and n is the refractive index of infrared ray absorbing part 315, the thickness of infrared ray absorbing part 315 is expressed by xcex/(4 n). The reflectance of infrared rays on metal reflecting film 313 is preferably 100%, and the sheet resistance of metal infrared ray absorbing film 316 is preferably on the order of 377xcexa9. By satisfying these conditions, infrared light of wavelength xcex is effectively absorbed and converted to heat by the optical resonation structure such as shown in FIG. 3. The converted heat is conveyed through junction pillar 314 to thermistor-bolometer thin-film 303 and is thereby changed to the resistance of thermistor-bolometer thin-film 303. Change in the resistance of thermistor-bolometer thin-film 303 is converted to voltage change by the signal read-out circuit of silicon substrate 300, outputted as an electrical signal, and this electrical signal is then converted to an image by an outside circuit.
As shown in FIG. 4, a concavity that is to become cavity 402 is formed on the surface of SOI (Silicon on insulator) silicon substrate 400 in the thermal infrared detector array described in the above-mentioned paper by Ishikawa et al. (SPIE Vol. 3698, 1999, pp. 556-564). Thermal detector 401 is arranged over this cavity 402. Thermal detector 401 is held by beams 405 above, and separated by the space of cavity 402 from, the bottom surface of cavity 402, i.e., separated from SOI silicon substrate 400.
Silicon diodes 403 are formed in a series on the SOI film of thermal detector 401, and silicon diodes 403 are surrounded by dielectric protective film 404. Embedded oxide film 413 is present on SOI silicon substrate 400. Silicon diodes 403 are electrically connected to signal line 407 on SOI silicon substrate 400 and to signal read-out circuit (not shown in the figure) in SOI silicon substrate 400 by metal wiring 406 that is formed in beam 405 for conveying current to silicon diodes 403. Metal wiring 406 is surrounded by dielectric film 408.
A structure composed by laminating infrared ray reflecting film 409, dielectric film 411, and infrared ray absorbing film 412 in that order is joined to the surface of thermal detector 401 that is directed away from SOI silicon substrate 400.
Of the structure that is formed by laminating infrared ray reflecting film 409, dielectric film 411, and infrared ray absorbing film 412, the portion at which thermal detector 401 is joined constitutes junction pillar 410 that projects toward thermal detector 401. The surface of infrared ray reflecting film 409 at junction pillar 410 contacts thermal detector 401. Infrared ray reflecting film 409, dielectric film 411, and infrared ray absorbing film 412 constitute an optical resonation structure of three-layer construction.
If xcex is the wavelength of infrared light that is to be detected by the thermal infrared detector (in concrete terms, the 8-12 xcexcm region) and n is the refractive index of dielectric film 411, the thickness of dielectric film 411 is represented by xcex/(4 n). Dielectric film 411 is constituted by a silicon oxide film and a silicon nitride film. The infrared reflectance of infrared ray reflecting film 409 is preferably 100%, and the sheet resistance of infrared ray absorbing film 412 is preferably on the order of 377 xcexa9. By satisfying these conditions, infrared light of wavelength xcex can be effectively absorbed and converted to heat by an optical resonation structure such as shown in FIG. 4. The converted heat is conveyed through junction pillar 410 to silicon diodes 403, and the current-voltage characteristics at silicon diodes 403 are changed by the conveyed heat. The change in the current-voltage characteristics at silicon diodes 403 is converted to a change in voltage by the signal read-out circuit and outputted as an electrical signal, and this electrical signal is converted to an image by an outside circuit.
The sensitivity of a thermal infrared detector increases with the degree of thermal isolation between the thermal detector and substrate. In the case of the infrared detector that is described in the above-cited paper by Ishikawa et al., the thermal conductance is as low as 8.2xc3x9710xe2x88x928 W/K, and a high sensitivity can be expected.
Referring now to FIGS. 5 and 6, in the thermal infrared detector described in Japanese Patent Laid-open Publication No. 185681/98 by Kimura et al., infrared photosensitive area 510 is supported above semiconductor substrate 504 by two beams 501, first column 502, and second column 503. As shown in FIG. 6, polycrystalline silicon film 511, which is a pn-junction thermistor, is formed on infrared photosensitive area 510. Infrared photosensitive area 510 is constituted by an infrared ray absorbing layer for absorbing the energy of incident infrared light and a thermo-electric conversion layer that functions as a detector. This thermo-electric conversion layer electrically detects the change in a physical value (for example, resistance) that is caused by the rise in temperature that is in turn brought about by the absorption of infrared energy in the infrared ray absorbing layer.
Each of the two beams 501 is formed as a plate with an L-shaped planar shape and is arranged between infrared photosensitive area 510 and semiconductor substrate 504. Impurity diffusion layer 504a that is formed in portions of the surface layer of semiconductor substrate 504 and one end of beam 501 are connected by first column 502; and the other end of beam 501 and infrared photosensitive area 510 are connected by second column 503. The supports are constituted by beam 501, first column 502 and second column 503. Infrared photosensitive area 510 is supported by these supports above semiconductor substrate 504 with an interposed gap M of prescribed height h shown in FIG. 6. Beam 501, first column 502 and second column 503 are thus arranged below infrared photosensitive area 510.
As shown in FIG. 6, infrared photosensitive area 510 is made up by: polycrystalline silicon film 511 and silicon nitride films 512, 513, and 514 that cover the surface of polycrystalline silicon film 511. Silicon nitride is composed of a material that easily absorbs infrared rays, and silicon nitride films 513 and 514 that are formed on the upper surface of polycrystalline silicon film 511 therefore determine the essential size (area) of the infrared ray absorbing layer on infrared photosensitive area 510.
An n-type diffusion layer and p-type diffusion layer are formed on polycrystalline silicon film 511, and this n-type diffusion layer and p-type diffusion layer constitute a pn-junction thermistor. In addition, through-hole 524 is provided at a prescribed position of infrared photosensitive area 510, and high-concentration impurity diffusion layer (conductor) 51 a is formed so as to surround through-hole 524. This high-concentration impurity diffusion layer 511a is electrically connected to the above-described pn-junction thermistor.
Each of two beams 501 is constituted by titanium film 515 and silicon nitride films 516 and 517 that cover titanium film 515, as shown in FIG. 6. of these films, one end of titanium film 515 is electrically connected to impurity diffusion layer 504a that is formed on semiconductor substrate 504. In addition, opening 517a is provided on silicon nitride film 517 that covers titanium film 515; and at this opening 517a, the other end 515b of titanium film 515 is electrically connected by way of aluminum film 518 to high-concentration impurity diffusion layer (conductor) 511a that is formed on polycrystalline silicon film 511. The conductive portion (or semiconductive portion) of infrared photosensitive area 510 and impurity diffusion layer 504a of semiconductor substrate 504 are thus electrically connected by way of titanium film 515 and aluminum film 518.
One end 515a of titanium film 515 functions as first column 502 (and a conductor), and aluminum film 518 functions as column 503 (and a conductor). Aluminum film 518 is formed on the inner walls of through-hole 524 so as to connect to high-concentration impurity diffusion layer 511a at through-hole 524. The outer surface and inner surface of aluminum film 518 are covered by silicon nitride films 514 and 513 as protective films.
When infrared light is irradiated upon infrared photosensitive area 510 in this type of thermal infrared detector, the incident infrared rays are absorbed into the infrared ray absorbing part of infrared photosensitive area 510 and converted to heat. A physical value (for example, resistance) of the detector portion of infrared photosensitive area 510 then changes in accordance with the amount of converted heat. As described hereinabove, beams 501 are arranged below infrared photosensitive area 510 and substantially parallel to infrared photosensitive area 510. In addition, first column 502 and second column 503 that together with beams 501 constitute the support structure are arranged below infrared photosensitive area 510. The support structure that is made up by beams 501, first column 502, and second column 503 is therefore covered by infrared photosensitive area 510 when viewed from the direction of incidence of infrared light (from above in FIG. 6), thereby enabling an increase in the proportion (the fill factor) of the area occupied by infrared photosensitive area 510 and allowing an improvement in thermal resolution.
The thermal infrared detectors described in the above-described Japanese Patent Laid-open Publication No. 209418/98 and the paper by Ishikawa et al. (SPIE Vol. 3698, 1999, pp. 556-564) have low thermal conductance and a high fill factor and thus can be expected to have high sensitivity. A thermal time constant sufficiently lower than 30 msec is necessary to obtain real-time imaging (a frame rate of at least 30 Hz) using a thermal infrared detector array. In each of the thermal infrared detectors shown in FIGS. 1-6, the thermal time constant (xcfx84h) can be represented by the ratio of the thermal capacity (H) of the thermal detector and infrared ray absorbing part to the thermal conductance (Gth) of the thermal isolation structure, as shown in the following equation (1):
xcfx84h=H/Gthxe2x80x83xe2x80x83(1)
In each of the thermal infrared detectors of the above-described prior art, the thermal time constant can be expected to be considerably greater than 30 msec as explained hereinbelow, and xe2x80x9cpersistence of visionxe2x80x9d can therefore be expected to pose a serious problem in real-time imaging.
In the case of the thermal infrared detector disclosed in the previously described paper by Ishikawa et al. (SPIE Vol. 3698, 1999, pp. 556-564), the thermal conductance value was described to be 8.2xc3x9710xe2x88x928 W/K, but no mention was made of thermal capacity. It can be understood from the SEM photograph in this paper that the size of the thermal detector is approximately 17xc3x9723 xcexcm. Although no mention is made of thermal conductance in the publication by Kimata et al. (Japanese Patent Laid-open Publication No. 209418/98 ), which is the same group as the above-described Ishikawa et al., the thickness of the thermal detector can be estimated to be approximately 1 xcexcm based on the thickness of the beams, and the thermal capacity of the thermal detector can therefore be calculated. In addition, the thermal capacity of the infrared ray absorbing part for the optical resonation structure of Ishikawa et al. is calculated based on the values of the refractive indices of the silicon oxide film and silicon nitride film in the wavelength band of 8-12 xcexcm and the specific heat at constant volume of the two materials. As described in the paper by Given W. Cleek (xe2x80x9cThe Optical Constants of Some Oxide Glasses in the Strong Absorption Regionxe2x80x9d; Applied Optics, vol. 5, No. 5, 1966, p, 771), the refractive index of silicon oxide film for infrared light in the 8-12 xcexcm wavelength region is in the range 0.51-3.38 and a unique absorption occurs at the 9.5 xcexcm wavelength region, and it is therefore difficult to determine a representative refractive index in the same wavelength region. Referring to FIG. 7 shown on p. 774 of the above-described paper by Given W. Cleek, however, 1.5 was used as the refractive index of a silicon oxide film for infrared light of the 8-12 xcexcm wavelength region. The refractive index of a silicon nitride film in the 8-12 xcexcm wavelength region is calculated as 1.9 based on the data for reflectance of a silicon nitride film shown in FIG. 7 of published Japanese translations of PCT International Publication No. 509057/95 by B. E. Cole. Depending on the method of film growth, the specific heat at constant volume of a siliconoxide film ranges from 1.05 J/cm3. K (Henry Baltes and Oliver Paul, xe2x80x9cThermal sensors Fabricated by CMOS and Micromachiningxe2x80x9d; Sensors and Materials, vol. 8, 1996, pp. 409-421) to 2.27 J/cm3xc2x7K (R. A. Wood, xe2x80x9cMonolithic Silicon Microbolometer Arraysxe2x80x9d in xe2x80x9cUncooled Infrared Imaging Arrays and Systems,xe2x80x9d Semiconductors and Semimetals, Volume 47, volume editors Paul W. Kruse and David D. Skatrud, Academic Press, 1997, p. 99). No data can be found for the specific heat at constant volume of a silicon nitride film. The thermal capacity is calculated using the value 1.7 J/cm3xc2x7K for both materials.
First, the thickness of dielectric film 411 of the infrared ray absorbing part is estimated to be 1.3-1.7 xcexcm based on the values of the refractive index of a silicon oxide film and a silicon nitride film for the 8-12 xcexcm wavelength region. If it is considered that the fill factor of the thermal infrared detector array of Ishikawa et al. is 90%, the thermal capacity of each of thermal detector 401 and the infrared ray absorbing parts (409, 411, and 412) are 6.6xc3x9710xe2x88x9210 J/K and (3.2-4.2)xc3x9710xe2x88x929 J/K, respectively, making the total thermal capacity (3.9-4.8)xc3x9710xe2x88x929 J/K. The thermal time constant of the thermal infrared detector array of Kimata et al. and Ishikawa et al. is estimated to be 47-58 msec based on these values and on the value of thermal conductance 8.2xc3x9710xe2x88x928 W/K, and persistence in real-time imaging is therefore judged to be problematic.
Next, in a case in which the infrared ray absorbing part is constituted by only dielectric film 411 composed of a silicon nitride film having a thickness of 500 nm and infrared ray reflecting film 409 composed of a titanium film having a thickness of 150 nm, the thermal capacity of the infrared ray absorbing part is 1.7xc3x9710xe2x88x929 J/K and the thermal time constant is calculated to be 30 msec. However, persistence is judged to be a problem because the thermal time constant still does not differ significantly from the television frame rate of 30 Hz (a time interval of 33 msec).
In the case of the thermal infrared detector of Japanese Patent Laid-open Publication No. 185681/98, first column 502 and second column 503 are necessary due to the three-layer structure of infrared photosensitive area 510, beam 501, and semiconductor substrate 504. The problem of poor contact of the conductive material therefore tends to arise. There is also the problem that the portion directly above and in the vicinity of second column 503 functions as the electrode of the thermistor, and the fill factor is therefore correspondingly reduced.
It is an object of the present invention to provide a thermal infrared detector that can realize an increase in sensitivity and raise the fill factor while at the same time bringing about almost no increase in the thermal time constant, and to provide a fabrication method of such a thermal infrared detector.
According to one aspect of the present invention, infrared light is irradiated upon an infrared ray absorbing part of an infrared photosensitive area that is held up above one surface of a substrate by supports and upon a shield that projects from the infrared ray absorbing part, whereby at least a portion of the incident infrared light is absorbed by the infrared ray absorbing part and shield to heat these components. The heat of the infrared ray absorbing part is conveyed to a thermal detector of the infrared photosensitive area and the heat of the shield is also conveyed through the infrared ray absorbing part to the thermal detector, whereby the temperature of the thermal detector changes. The change in temperature of the thermal detector is transmitted as a signal to, for example, a signal read-out circuit by way of electrodes that are electrically connected to the thermal detector, the wiring of the support, and contact pads of the substrate, and this signal is converted to an electrical signal by the signal read-out circuit. The temperature change of the thermal detector is then converted to an infrared image by, for example, an outside circuit based on the electrical signal. Here, the fill factor of each picture element of the thermal infrared detector can be increased and more infrared light can be absorbed by causing the shield to project from the infrared ray absorbing part of the infrared photosensitive area such that this shield covers the surface of the electrodes that is directed away from the substrate with a space interposed between the electrodes and the shield. This configuration enables an increase in the sensitivity of the thermal infrared detector. Furthermore, arranging the base of the shield in proximity to the edge of the infrared photosensitive area can make the thermal time constant sufficiently lower than the time interval of 33 msec that corresponds to the television frame rate, while bringing about almost no increase in thermal capacity. In other words, the present invention realizes a thermal infrared detector that is capable of real-time infrared imaging with higher sensitivity.
In addition, the shield that projects from the infrared ray absorbing part preferably covers the surfaces of the supports that are directed away from the substrate and contact pads with an interposed space between the shield and both the supports and the contact pads of the substrate. This configuration can further increase the fill factor of each picture element in the thermal infrared detector and realize greater absorption of infrared light.
According to another aspect of the present invention, the shield projects from portions other than portions that correspond to the electrodes in the infrared ray absorbing part of the infrared photosensitive area; such that this shield covers the supports and the contact pads of the substrate. This configuration enables an increase in the fill factor of each picture element of the thermal infrared detector and increases the absorption of infrared light. Here, the heat of the shield can be prevented from escaping to the substrate by way of the electrodes and wiring of the supports by causing the shield to project from portions of the infrared ray absorbing part other than the portion corresponding to electrodes. This configuration therefore prevents deterioration in the sensitivity of the thermal infrared detector.
Each of the above-described thermal infrared detectors preferably further includes: an infrared ray reflecting film that is formed on the surface of the substrate that is directed toward the infrared photosensitive area, and a first dielectric protective film that is formed on the surface of the infrared ray reflecting film so as to cover the infrared ray reflecting film. In this case, the infrared photosensitive area is held up from the first dielectric protective film by supports above the first dielectric protective film. In this configuration, infrared light that has been transmitted by the infrared photosensitive area is reflected toward the infrared photosensitive area by the infrared ray reflecting film on the substrate. The reflected infrared light is again irradiated upon the infrared ray absorbing part and shield and absorbed by these components. Thus, the formation of the infrared ray reflecting film on the surface of the substrate allows more infrared light to be absorbed by the infrared ray absorbing part and shield.
In the above-described configuration, at least a portion of the infrared ray absorbing part is arranged on the surface of the thermal detector that is directed away from the substrate, and the thermal infrared detector includes a metal thin-film that is formed on the surface of the infrared ray absorbing part that is directed away from the substrate and on the surface of the shield that is directed away from the substrate. This formation of a metal thin-film on the surfaces of the infrared ray absorbing part and shield realizes a construction in which infrared rays mutually interfere on the metal thin-film and heat the metal thin-film. As to the actual operation in a thermal infrared detector of this type of configuration, first, when infrared light is irradiated upon the metal thin-film on the infrared ray absorbing part and the shield, a portion of the incident infrared light is reflected by the metal thin-film. The rest of the infrared light that is incident to the metal thin-film passes through the metal thin-film and proceeds toward the substrate. The infrared light that has passed through the metal thin-film is then reflected toward the metal thin-film by the infrared ray reflecting film and contact pads on the substrate and again irradiated upon the metal thin-film. The infrared light that is again irradiated upon the metal thin-film causes destructive interference with the original infrared light that is to be reflected by the metal thin-film, and the infrared rays that cause interference are together absorbed by free electrons in the metal thin-film to become heat. The metal thin-film is consequently heated, its temperature rises, and the heat of the metal thin-film is transmitted by way of the shield and infrared ray absorbing part to the thermal detector. In this case, the detector is configured such that the heat of the shield and infrared ray absorbing part is rapidly conveyed to the thermal detector by the metal thin-film that is formed on the shield and infrared ray absorbing part.
The substrate preferably includes read-out circuits that: are electrically connected to the contact pads, convert the temperature change of the infrared ray absorbing part that is detected by the thermal detector to an electrical signal, and read out this electrical signal.
According to yet another aspect of the present invention, not only does the shield cover the supports of the infrared ray absorbing part and the contact pads of the substrate, but the shield further covers the surfaces of the electrodes of the infrared ray absorbing part that are directed away from the substrate with an interposed space. As with the above-described cases, this type of configuration enables an increase in the fill factor of each picture element of the thermal infrared detector and allows the absorption of more infrared light.
According to yet another aspect of the present invention, a shield such as is described hereinabove is provided in a configuration in which a metal thin-film, which is provided in an infrared photosensitive area such as described hereinabove, is heated by infrared rays that are caused to mutually interfere at the metal thin-film. The metal thin-film of the infrared ray absorbing part extends over the entire surface of the shield that is directed away from the substrate. When infrared light is irradiated upon the metal thin-film of the infrared photosensitive area in this thermal infrared detector, a portion of the irradiated infrared light is reflected by the metal thin-film. The remaining portion of the infrared light that is irradiated upon the metal thin-film passes through the metal thin-film and proceeds toward the substrate. The infrared light that has passed through the metal thin-film is reflected toward the metal thin-film by the infrared ray reflecting film and contact pads on the substrate and is again irradiated upon the metal thin-film. In this case, the infrared light that is again irradiated upon the metal thin-film causes destructive interference with the original infrared light that is to be reflected by the metal thin-film, and the infrared rays that produce this interference are absorbed by free electrons in the metal thin-film to become heat. The metal thin-film is therefore heated and its temperature rises, and the heat of the metal thin-film is conveyed to a thermal detector by way of the shield and dielectric film that contact the metal thin-film. In this thermal infrared detector that is configured such that infrared rays mutually interfere on a metal thin-film and heat the metal thin-film, the shield is caused to project from the dielectric film of the infrared photosensitive area, whereby this shield covers the electrodes of the infrared photosensitive area, the supports, and contact pads of the substrate. Extending the metal thin-film over the entire surface of the shield that is directed away from the substrate increases the fill factor of each picture element of the thermal infrared detector and enables greater absorption of infrared light, whereby the sensitivity of the thermal infrared detector can be increased.
The thermal detector of the infrared ray absorbing part is preferably any one of a thermistor-bolometer thin-film, a pyroelectric thin-film, or a thermopile.
Finally, the fabrication method of the thermal infrared detector of the present invention can produce a thermal infrared detector that has higher sensitivity and a higher fill factor and that can absorb more infrared light, as described hereinabove.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.